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Simulation of a Steam Coal Gasifier

Presented by

Alireza Abbasi1-3

Paul E. Ege2, Hugo I. deLasa1

(1)Department of Chemical and Biochemical Engineering, University of Western Ontario, London, ON, Canada

(2) Reactech Process Development Inc., Markham, ON, Canada

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Presentation outline

1- Introduction -Fluidized Bed Modeling

2 - Model -CFD and Plug Flow

3 - Results

4 - Summary

Fluidized Bed Properties and Applications

Properties• Excellent for contacting large gas volumes

effectively with high solid surface area at near isothermal conditions

• High relative gas/solid velocities • Strong particle mixing

Applications • Catalytic processes

• Steam Methane Reforming• Gasification• Catalytic Cracking

• CVD processes• Silicon deposition (SiH4/SiHCl3)• Uranium coating• TiO4 coating• Potassium nitrate granulation

• Other• Combustion (coal/biomass)• Thermal Gasification• Drying

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Introduction

Modeling Principles

Essentially, all models are wrong, but some are useful(George E. P. Box, 1987)

All the physics is not understood

There are lots of assumptions in calculation

Simplified Approach Models

Pseudo Homogeneous • Ideal flow (PF/CSTR), • Dispersion models, • RTD or CTD models• Simplified flow • Single phase assumption

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Introduction

Two-phase modeling • More advanced = account for bubble/emulsion• Gas in excess of minimum fluidization = bubbles• Two distinct phases: bubble & emulsion• Each phase with has a model for flow & reactions• Mass interchange between phases

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1-The Eulerian-Lagrangian approach: The fluid phase is treated as a continuum by solving the time-averaged Navier- Stokes equations, while the dispersed phase is solved by tracking a large number of particles through the calculated flow field. (CPFD)

2-The Eulerian-Eulerian approach: It solves a set of n momentum and continuity equations for each phase. Couplings are achieved through the pressure and inter phase exchange coefficients. (CFD)

CPFD CFD

Computational Fluid Dynamics (CFD)

Introduction

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Modeling a Coal Gasification Fluidized-Bed Reactor

Model

(a) The schematic representation of the entrained fluidized bed gasifier. (b) Selected configuration for the simulation of the entrained fluidized bed gasifier in the near feeding section.

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Modeling a Coal Gasification Fluidized-Bed Reactor

Model

AshOHVolatileCharCoal +++→ 2

24322412 COCOHCHOHVolatile ααααβ +++→+

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Mathematical Modeling

Model

CFDThe motion of fluid and dispersed phases are governed by respective mass and momentum conservation equations. The volume averaged fluid mass, momentum and energy equations are defined as follows:

The trajectory calculation of the discrete phase is made by integrating the force balance on the particles. The particle motion is defined as follows:

(1)

(2)

(3)

(4)

Plug FlowA material balance on the differential volume of a fluid element on species i in a PFR is calculated as follows:

(5)

(6) 0reaction ofheat

3

leaving species

2

coming species

1 QHHH =∆+∆+∆

( )

action

j jr

jji

cxi

RvAF

dx

d

Re

,

,

Changing flowmolar

, ∑=ν

( ) ( )

Source

f

Convection

fff

ChnageDensity

ff Svt

=⋅∇+∂∂

θρθρ

( ) ( )

ExchangeMementum

ForceBody

ff

Shear

ffessure

Convection

ffff

onAccelerati

fff FgPvvvt

−+⋅∇+∇−=⋅∇+∂∂

θρτθθρθρ

Pr

( ) ( )

ForceBody

Stress

pp

p

essure

pForceDrag

pfp

onAccelerati

p gP

vvDvdt

d +⋅∇

−∇−−=

ρθτ

ρPr

( ) ( )[ ] SourceConductionConvection

fff

onAccumulati

ff QqvPEEt

∗+∇−=+⋅∇+

∂∂

.θρθρ

CFD Modeling

Particle contours after 10S

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Results

Fluid contours after 10S

Results

CFD Modeling

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Gas species contour after 10s

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Results

CFD Modeling

Gas species contour after 10s

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Results

CFD Modeling

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Results

Effluent concentrations

CFD Modeling

Particle recycle flow at 1350KParticle recycle flow at 1150K

Plug Flow Modeling

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Results

Axial Concentration profile

Particle recycle flow at 1350KParticle recycle flow at 1150K

Modeling Results

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Results

Temperature Profile

CFD and Plug Flow Comparison

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Results

Effluent concentrations

Particle recycle flow at 1350KParticle recycle flow at 1150K

Summary

Reasonable agreement between CFD and PF effluents Methods are complementary, fast general or slow detailed PF model set up in few days allow quick investigation

Useful for wide range of conditions (height, density, flow, temp, pressure…)

Possible to apply different kinetics (even parameter fit to experiments)

Useful for feasibility, optimization and In Situ calculations ,fast basic understanding of process

CFD takes time to set up and run different cases Detail understanding of bed hydrodynamics Investigate flow/species patterns Specify flow related issues (hot spot, bypass) Benefit for entrainment and choking Useful for scale-up and design Also verify changes prior to implementing in practice

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